.beta..sub.2 -microglobulin is a light chain of a double-stranded protein constituting the major histocompatibility antigen (in case of human, it is HLA, class I), and occurs on the surfaces of most of cells. It also occurs in the body fluid in the free form, but the physiological function of the free .beta..sub.2 -microglobulin has not yet been known. The full amino acid sequence thereof has been determined for human and other various animals, and its three dimensional structure has been determined by X-ray analysis for bovine. It has been proved that it is a simple protein with a molecular weight of about 12,000, which does not have a sugar chain, and that it has structurally high homology with the C domain (constant domain) of immunogulobulin. Further, the homology of the amino acid sequence thereof between different species is 60 to 80%, and thus it is considerably high (Proc. Natl. Acad. Sci. 257, 2619 (1982)).
The .beta..sub.2 -microglobulin level in the blood of the patients suffering from nephropathy, who are undergoing artificial blood dialysis for a long period, is as high as 10 to 100 times that of normal human. It is assumed that this is because that .beta..sub.2 -microglobulin which is decomposed in the kidney is not removed by the blood dialysis and thus accumulates in the blood.
The present inventors separated and analyzed the amyloid proteins deposited on the diseased part of a patient suffering from carpal tunnel syndrome to find that most of the amyloid proteins are .beta..sub.2 -microglobulin. Thus, it is assumed that carpal tunnel syndrome is caused by the deposition of the .beta..sub.2 -microglobulin on the diseased part, which is accumulated in the blood with high level. Thus, it is expected that carpal tunnel syndrome can be prevented by removing the .beta..sub.2 -microglobulin in the blood along with the artificial blood dialysis. Further, it is possible that .beta..sub.2 -microglobulin is involved in the deposition of amyloid on the parts other than the carpal tunnel.
Heretofore, no disease has been known of which cause is clarified to be the .beta..sub.2 -microglobulin in the blood, and so how to remove the .beta..sub.2 -microglobulin in the blood has not been considered at all.